1. Field of the Invention
This invention relates to semiconductive structures made from silicon germanium. More specifically, the invention is a lattice matched silicon germanium semiconductive alloy having at least one graded index silicon germanium layer deposited thereon.
2. Description of the Related Art
Silicon germanium (SiGe) is an important semiconductive alloy used in Field Effect Transistors (FETs), High Electron Mobility Transistors (HEMTs), Hetero Bi-polar Transistors (HBTs), Thermo-Electric (TE) devices, photovoltaic solar cells, and photon detectors. Typically, SiGe is grown on common silicon wafer substrates. However, the growth of thick (i.e., greater than tens of nanometers), high-quality (i.e., defect free) SiGe layers on a silicon substrate is difficult because SiGe has a larger lattice constant than silicon. This means that the SiGe layers close to the silicon substrate are strained while severe defects (e.g., threading dislocations, micro twins, cracks, delaminations, etc.) develop in the layers of SiGe that exceed the critical thickness of hundreds of nanometers because of the lattice mismatch. Thus, at best, only strained SiGe layers with very thin thicknesses (i.e., less than hundreds of nanometers) are utilized for high-quality electronic device fabrication.
Recently, methods for making thicker high-quality SiGe semiconductive alloys were disclosed in U.S. patent application Ser. No. 11/242,415, filed Sep. 27, 2005, the entire contents of which are hereby incorporated by reference. In this application, a lattice matched SiGe semiconductive alloy and method of fabricating same are provided. Briefly, the {0001} C-plane of a substrate of single crystalline Al2O3 (i.e., sapphire) is exposed and a {111} crystal plane of a cubic diamond structure SiGe is grown on the {0001} C-plane such that a <110> orientation of the cubic diamond structure SiGe is aligned with a <1,0,−1,0> orientation of the {0001} C-plane. A lattice match between the single crystalline Al2O3 and the cubic diamond structure SiGe is achieved by using a SiGe composition that is 0.7223 atomic percent silicon and 0.2777 atomic percent germanium. A strained layer can be grown on top of the lattice matched SiGe. The composition of the strained layer can be pure silicon, pure germanium, or a silicon germanium composition of the general form Si1-YGeY where Y is the atomic percent of germanium that satisfies the relationship 0.0<Y<1.0. However, the strained layer presents an abrupt transition on the lattice matched SiGe that causes
(i) misfit dislocations,
(ii) an electric potential barrier at the interface between the lattice matched SiGe and the strained layer,
(iii) an abrupt change in refractive index,
(iv) an abrupt bandgap change,
(v) generation of interfacial mid-bandgap states, and
(vi) abrupt strain build-up that tends to attract other atoms to thereby introduce impurities into the device.